This invention relates to a method for the preparation of a stable sodium carbonate peroxide. More particularly, an ethylene oxide-derived stabilizing agent is incorporated into sodium carbonate peroxide to stabilize the dry, free-flowing sodium carbonate peroxide solid against loss of its active oxygen.
Sodium carbonate peroxide, alternatively known as sodium carbonate peroxyhydrate or sodium percarbonate, has the formula Na.sub.2 CO.sub.3 11/2H.sub.2 O.sub.2 and is typically a white, free-flowing crystalline solid. Because sodium carbonate peroxide releases hydrogen peroxide when placed in aqueous solution, this peroxygen compound is useful as a bleaching and cleansing agent, typically in household detergent formulations.
Many processes are described in the prior art for the preparation of sodium carbonate peroxide (occasionally abbreviated as SCP in this disclosure) from the basic reaction of hydrogen peroxide with sodium carbonate. These include the "dry" process disclosed in U.S. Pat. No. 3,860,694, wherein hydrogen peroxide is reacted with moist sodium carbonate in particulate form and the reaction mass is thereafter dried to yield the SCP product, and the "wet" process such as disclosed in U.S. Pat. Nos. 3,870,783 and 3,801,706, wherein aqueous solutions of sodium carbonate and hydrogen peroxide are mixed in a reactor and the sodium carbonate peroxide is crystallized from solution, thereafter being recovered as a dry SCP product. These prior art methods include both continuous and batch methods for the preparation of sodium carbonate peroxide.
A common drawback associated with all of these prior art processes is the instability of the sodium carbonate peroxide crystallized solid resulting in loss of its active oxygen content. Heavy metal impurities, such as iron, copper, magnesium, and lead, are believed responsible for catalyzing the decomposition of sodium carbonate peroxide. These metallic contaminants are typically present in the sodium carbonate raw material or may occasionally be introduced into the SCP product from the process equipment.
The use of stabilizing agents to minimize the heavy metal-accelerated decomposition of sodium carbonate peroxide is well established in the prior art. Several approaches are taught in the prior art for utilizing stabilizing agents in the preparation of a stable sodium carbonate peroxide. In one such method, described in British Pat. No. 1,398,876, the stabilizing agent (mono- or di-fatty ester of polyalkylene glycol or amide of amine-terminated polyalkylene glycol) is applied as a coating to the dry, crystalline SCP product. This method, however, is ineffective against decomposition occurring in the SCP crystals formed during their preparation by the wet process and can only be used to stabilize SCP after it has been recovered and dried.
A second procedure, described in U.S. Pat. No. 3,870,783, treats the raw materials rather than the final dry SCP product. In this method, heavy metal contaminants present in the aqueous sodium carbonate reactant are complexed and passivated by treatment with a soluble magnesium salt or magnesium oxide prior to reaction of the sodium carbonate with hydrogen peroxide.
A third approach, utilized in U.S. Pat. Nos. 3,677,697, 3,801,706, 3,860,694 and 3,870,783 involves the reaction of sodium carbonate with hydrogen peroxide in the presence of one or more stabilizing agents. These prior art patents teach that suitable stabilizing agents for use in this method include such diverse compounds as water soluble magnesium salts, sodium silicate, magnesium silicate, ethylenediamine tetraacetic acid and benzoic acid.
The ability of a particular compound or material to function as an effective stabilizing agent is unpredictable, as evidenced by the diverse nature of the group of compounds noted above. For example, some sequestering agents (sodium pyrophosphate) may be utilized as stabilizing agents, but the mere fact that a compound is a sequestering agent does not ensure that it will perform effectively as a stabilizing agent for sodium carbonate peroxide. The absence of this prior art of specific knowledge regarding the physical and/or chemical mechanism by which stabilizers for SCP function is one factor that contributes to the inability to predict which compositions will yield superior results as stabilizing agents for SCP.